In the related art magnetic recording technology such as hard disk drives, a head is equipped with a reader and a writer. The reader and writer have separate functions and operate independently of one another.
FIGS. 1 (a) and (b) illustrate related art magnetic recording schemes. In FIG. 1(a), a recording medium 1 having a plurality of bits 3 and a track width 5 has a magnetization parallel to the plane of the recording media. As a result, a magnetic flux is generated at the boundaries between the bits 3. This is also commonly referred to as “longitudinal magnetic recording media” (LMR).
Information is written to the recording medium 1 by an inductive write element 9, and data is read from the recording medium 1 by a read element 11. A write current 17 is supplied to the inductive write element 9, and a read current is supplied to the read element 11.
The read element 11 is a magnetic sensor that operates by sensing the resistance change as the sensor magnetization direction changes from one direction to another direction. A shield 13 reduces the undesirable magnetic fields coming from the media and prevents the undesired flux of adjacent bits from interfering with the one of the bits 3 currently being read by the read element 11.
The area density of the related art recording medium 1 has increased substantially over the past few years, and is expected to continue to increase substantially. Correspondingly, the bit and track densities are expected to increase. As a result, the related art reader must be able to read this data having increased density at a higher efficiency and speed.
In the related art, the density of bits has increased much faster than the track density. However, the aspect ratio between bit size and track size is decreasing. Currently, this factor is about 8, and is estimated to decrease to 6 or less as recording density approaches terabyte size.
As a result, the track width is becoming so small that the magnetic field from the adjacent tracks, and not just the adjacent bits, will affect the read sensor. Table 1 shows the estimated scaling parameters based on these changes.
TABLE 1Arealbittrackbit aspectbitread trackTrackDensitydensitydensityratiolengthwidthpitchGbpsi(Mbpi)(ktpi)(bit/track)nmnmnm2001.21607.5201001504001.82228.114.17611060023006.712.755 8510002.53806.59.745~?
Another related art magnetic recording scheme has been developed as shown in FIG. 1(b). In this related art scheme, the direction of magnetization 19 of the recording medium 1 is perpendicular to the plane of the recording medium. This is also known as “perpendicular magnetic recording media” (PMR)
This PMR design provides more compact and stable recorded data. However, with PMR media the transverse field coming from the recording medium, in addition to the above-discussed effects of the neighboring media tracks, must also be considered. This effect is discussed below with respect to FIG. 6
The flux is highest at the center of the bit, decreases toward the ends of the bit and approaches zero at the ends of the bit. As a result, there is a strong transverse component to the recording medium field at the center of the bit, in contrast to the above-discussed LMR scheme, where the flux is highest at the edges of the bits.
FIGS. 2(a)-(c) illustrate various related art read sensors for the above-described magnetic recording scheme, also known as “spin valves”. In the bottom type spin valve illustrated in FIG. 2(a), a free layer 21 operates as a sensor to read the recorded data from the recording medium 1. A spacer 23 is positioned between the free layer 21 and a pinned layer 25. On the other side of the pinned layer 25, there is an anti-ferromagnetic (AFM) layer 27.
In the top type spin valve illustrated in FIG. 2(b), the position of the layers is reversed. FIG. 2(c) illustrates a related art dual type spin valve. Layers 21 through 25 are substantially the same as described above with respect to FIGS. 2(a)-(b). An additional spacer 29 is provided on the other side of the free layer 21, upon which a second pinned layer 31 and a second AFM layer 33 are positioned. The dual type spin valve operates according to the same principle as described above with respect to FIGS. 2(a)-(b).
In the read head based on the MR spin valve, the magnetization of the pinned layer 25 is fixed by exchange coupling with the AFM layer 27. Only the magnetization of the free layer 21 can rotate according to the media field direction.
In the recording media 1, flux is generated based on polarity of adjacent bits. If two adjoining bits have negative polarity at their boundary the flux will be negative, and if those bits have positive polarity at the boundary the flux will be positive. The magnitude of flux determines the angle of magnetization between the free layer and the pinned layer.
When the magnetizations of the pinned and free layers are in substantially the same direction, then the resistance is low. On the other hand, when their magnetizations are in opposite directions the resistance is high. In the MR head application, when no external magnetic field is applied, the free layer 21 and pinned layer 25 have their magnetizations at 90 degrees with respect to each other.
When an external field (flux) is applied to a reader, the magnetization of the free layer 21 is altered, or rotated by an angle. When the flux is positive the magnetization or the free layer is rotated upward, and when the flux is negative the magnetization of the free layer is rotated downward. Further, when the applied external field results in the free layer 21 and the pinned layer 25 having the same magnetization direction, then the resistance between the layers is low, and electrons can more easily migrate between those layers 21, 25.
However, when the free layer 21 has a magnetization direction opposite to that of the pinned layer 25, the resistance between the layers is high. This increased resistance occurs because it is more difficult for electrons to migrate between the layers 21, 25.
Similar to the external field, the AFM layer 27 provides an exchange coupling field and keeps the magnetization of pinned layer 25 fixed. In the related art, the AFM layer 27 is usually PtMn or IrMn.
The resistance change ΔR between the states when the magnetizations of layers 21, 25 are parallel and anti-parallel should be high to have a highly sensitive reader. As head size decreases, the sensitivity of the reader becomes increasingly important, especially when the magnitude of the media flux is decreased. Thus, there is a need for high resistance change ΔR between the layers 21, 25 of the related art spin valve.
FIG. 6 graphically shows the foregoing principle for the related art longitudinal magnetic recording scheme illustrated in FIG. 1(a). As the media spins, the flux at the boundary between bits acts on the free layer such that magnetization rotates upward and downward according to the related art spin valve principles.
The flux generated by the recording media results in a change in the magnetization direction of the free layer. As a result, an angle between the directions of magnetization of the free layer and the pinned layer is generated. The output signal of the reader is a function of the cosine of this angle. To increase the output signal, it is desirable to have a free layer that has a single magnetic domain. Such a configuration can cancel noise, more specifically known as Barkhausen noise that originates in non-oriented domains of the free layer.
U.S. Patent publication nos. 2002/0167768 and 2003/0174446, the contents of which are incorporated herein by reference, disclose side shields to avoid flux generated by adjacent tracks, along with an in-stack bias. This in-stack bias, or alternatively, a hard bias can reduce the effect of the above-described non-oriented domains. These related art bias and/or stabilizing schemes are discussed in greater detail below.
As shield-to-shield spacing declines below about 40 nm, it is difficult to avoid current leakage from the shield to the MR element. Further, as the head size decreases the field induced by sensing current will generate a vortex at the free layer.
FIGS. 8(a) and 8(b) illustrate the related art hard bias stabilizer and in-stack bias stabilizer, respectively. As shown in FIG. 8(a), the hard bias stabilizer includes an insulator 50 positioned on the bottom shield and the read sensor elements, a buffer 51 mounted on the insulator 50, and a hard bias layer 52 mounted on the buffer 51. In the related art, the hard bias layer 52 is made of CoCrPt.
In the related art hard bias, the buffer 51 must be thick in order to obtain a sufficiently large coercivity (e.g., greater than about 1000 Oe). More specifically, the buffer may include at least two or three different kinds of films, such that the total thickness of the buffer and insulator is 10 nm. The hard bias layer 52 and the buffer 51 are thick in the related art.
As a result of these layers 50, 51, the hard bias layer 52 is away from the free layer edge, which results in reduced stability due to the reduced hard bias field strength. Further, because the sides of the hard bias layer 52 grow in an oblique manner, the magnetic field induced by the hard bias will have not have the same easy direction as the easy axis of the free layer. This deviation between the free layer easy axis and the easy axis of the hard bias results in a less efficient hard bias. Further, due to its larger thickness, performance of the buffer is better at region A.
Additionally, because the hard bias layer 52 is made of CoPtX, where X is Cr and Ta, it is necessary to have a thicker hard bias layer to obtain the required coercivity. For example, a CoPtCr layer has a thickness of about 100 nm, in additional to the thick buffer layer required in this related art device.
Due to the oblique growth and its thinner buffer, coercivity is lower and performance is reduced at region B. Also, in the related art stabilizer, noise is generated (i.e., a vortex effect) in the free layer due to magnetic fields generated by sense current.
Accordingly, there is an unmet need in the related art to overcome the foregoing related art problems. For example, but not by way of limitation, there is an unmet need to reduce thickness of the stabilizer including the buffer layer 51. However, this cannot be done with the presently used materials due to the coercivity requirements of the MR sensor.
FIG. 8(b) illustrates a related art in-stack bias. In addition to the above-disclosed related art elements in FIGS. 1-7, a non-magnetic decoupler 61 is provided above the free layer, and a ferromagnetic stabilizer 62 is provided on the decoupler 61. Further, a second AFM layer 63 is provided on the stabilizer 62. The in-stack bias and sensor layers have an insulator 64 on their sides.
However, the foregoing related art in-stack bias has various problems and disadvantages. For example, but not by way of limitation, because the width of the free layer of the sensor exceeds the width of the in-stack bias elements 61-63, regions C of the structure are not well pinned. Thus, stability is reduced in at least those areas.
Because the in-stack bias is substantially smaller than the free layer located below, the magnetic domain at the edge of the free layer is not completely aligned with the easy axis.
Accordingly, the related art bias has various problems and disadvantages. For example, but not by way of limitation, when the free layer has a width of less than 100 nm, the magnetic moments are randomly distributed at the edge, which is a source of noise in region C. The free layer region below the noise source region C is not stabilized. Thus, undesired magnetic fluctuation is generated.
As the width of the free layer 21 decreases, the demagnetizing field increases. For example, the magnetization of the free layer may begin to switch at the edge of the free layer and extend toward the center of the free layer. Further fluctuations of magnetization accelerate this switching process.
Additionally, ion milling can damage the free layer edge. Further, the in-stack bias that uses the anti-ferromagnetic (AFM) layer 63 to pin the stabilizer layer is shorter than the stabilizer layer. As a result, the stabilizer layer is not fully pinned, and cannot provide the maximum stability.
As a result of the foregoing related art problems, there is a need to shield the bit from the flux generated at adjacent tracks as well as adjacent bits within a track.
In addition to the foregoing related art spin valve in which the pinned layer is a single layer, FIG. 3 illustrates a related art synthetic spin valve. The free layer 21, the spacer 23 and the AFM layer 27 are substantially the same as described above. In FIG. 3 only one state of the free layer is illustrated. However, the pinned layer further includes a first sublayer 35 separated from a second sublayer 37 by a spacer 39.
In the related art synthetic spin valve, the first sublayer 35 operates according to the above-described principle with respect to the pinned layer 25. Additionally, the second sublayer 37 has an opposite spin state with respect to the first sublayer 35. As a result, the pinned layer total moment is reduced due to anti-ferromagnetic coupling between the first sublayer 35 and the second sublayer 37. A synthetic spin-valve head has a pinned layer with a total flux close to zero, high resistance change ΔR and greater stability.
FIG. 4 illustrates the related art synthetic spin valve with a shielding structure. As noted above, it is important to avoid unintended magnetic flux from adjacent bits from being sensed during the reading of a given bit. A top shield 43 is provided on an upper surface of the free layer 21. Similarly, a bottom shield 45 is provided on a lower surface of the AFM layer 27. The effect of the shield system is shown in and discussed with respect to FIG. 6.
As shown in FIGS. 5(a)-(d), there are four related art types of spin valves. The type of spin valve structurally varies based on the structure of the spacer 23.
The related art spin valve illustrated in FIG. 5(a) uses the spacer 23 as a conductor, and is used for the related art CIP scheme illustrated in FIG. 1(a) for a giant magnetoresistance (GMR) type spin valve where the current is in-plane-to the film.
In the related art GMR spin valve, resistance is minimized when the magnetization directions (or spin states) of the free layer 21 and the pinned layer 25 are parallel, and is maximized when the magnetization directions are opposite. As noted above, the free layer 21 has a magnetization direction that can be changed. Thus, perturbation of the head output signal can be avoided by minimizing the undesired change of the pinned layer magnetization direction.
The MR ratio depends on the degree of spin polarization of the pinned and free layers, and the angle between their magnetizations. Spin polarization depends on the difference between the number of electrons in spin state up and down normalized by the total number of the conduction electrons in each of the free and pinned layers.
As the free layer 21 receives the flux that signifies bit transition, the free layer magnetization rotates by a small angle in one direction or the other, depending on the direction of flux. The change in resistance between the pinned layer 25 and the free layer 21 is proportional to angle between the magnetizations of the free layer 21 and the pinned layer 25.
The GMR spin valve has various requirements. For example, but not by way of limitation, a large resistance change ΔR is required to generate a high output signal. Further, low coercivity is desired, so that small media fields can also be detected. With high pinning field strength, the pinned layer magnetization direction is fixed against external magnetic field, and when the interlayer coupling is low, the sensing layer (free layer) is not affected by the pinned layer. Further, low magnetistriction is desired to minimize stress on the free layer.
In order to increase the recording density, the track width of the GMR sensor must be made smaller. In this aspect read head operating in CIP scheme (current-in-plane), various issues arise as the size of the sensor decreases. The magnetoresistance (MR) in CIP mode is generally limited to about 20%. When the electrode connected to the sensor is reduced in size overheating results and may potentially damage the sensor, as can be seen from FIG. 7(a). Further, the signal available from CIP sensor is proportional to the MR head width.
To address the foregoing issues and as shown in FIG. 7(b), related art CPP-GMR scheme uses a sense current that flows in a direction perpendicular to the spin valve plane. As a result, size can be reduced. Various related art spin valves that operate in the CPP scheme are illustrated in FIGS. 5(b)-(d), and are discussed in greater detail below.
FIG. 5(b) illustrates a related art tunneling magnetoresistive (TMR) spin valve for a CPP scheme. In the TMR spin valve, the spacer 23 acts as an insulator, or tunnel barrier layer. Thus, electrons can tunnel from free layer to pinned layer through the insulator barrier 23. TMR spin valves have an increased MR on the order of about 30-50%.
FIG. 5(c) illustrates a related art CPP-GMR spin valve. While the general concept of GMR is similar to that described above with respect to CIP-GMR, the current flows perpendicular to the plane, instead of in-plane. As a result, the resistance change DR and the intrinsic MR are substantially higher than the CIP-GMR.
In the related art CPP-GMR spin valve, there is a need for a large ΔR*A (A is the area of the MR element) and a moderate head resistance. A low free layer coercivity is required so that a small media field can be detected. The pinning field should also have a high strength.
FIGS. 7(a)-(b) illustrate the structural difference between the CIP and CPP GMR spin valves. As shown in FIG. 7(a), there is a hard bias 998 on the sides of the GMR spin valve, with an electrode 999 on upper surfaces of the GMR. Gaps 997 are also required. As shown in FIG. 7(b), in the CPP-GMR spin valve, an insulator 1000 is deposited at the side of the spin valve that the sensing current can only flow in the film thickness direction. Further, no gap is needed in the CPP-GMR spin valve.
As a result, the sense current has a much larger surface through which to flow, and thus, the overheating issue is substantially solved.
FIG. 5(d) illustrates the related art ballistic magnetoresistance (BMR) spin valve. In the spacer 23, which operates as an insulator, a ferromagnetic layer region 47 connects the pinned layer 25 to the free layer 21. The area of contact is on the order of few nanometers. As a result, there is a substantially higher MR due to electrons scattering at the domain wall created within this nanocontact. Other factors include the spin polarization of the ferromagnets, and the structure of the domain that is in nano-contact with the BMR spin valve.
However, the related art BMR spin valve is in early development. Further, for the BMR spin valve the nano-contact shape and size controllability and stability of the domain wall must be further developed. Additionally, the repeatability of the BMR technology is yet to be shown for high reliability.
In the foregoing related art spin valves of FIGS. 5(a)-(d), the spacer 23 of the spin valve is an insulator for TMR, a conductor for GMR, and an insulator having a magnetic nano-sized connector for BMR. While related art TMR spacers are generally made of more insulating materials such as alumina, related art GMR spacers are generally made of more conductive metals, such as copper.
Accordingly, there is a need to address at least the foregoing issues of the related art.